System having a superconductive cable

ABSTRACT

A system is provided having a superconductive cable (KA) which consists of a superconductive inner conductor ( 1 ), a screen arranged concentrically therewith and a dielectric applied ( 3 ) between the inner conductor and the screen. The screen (S) is constructed from a superconductive part ( 4 ) and a part ( 5 ) consisting of an electrically highly conductive material enclosing the latter, and in which the screen is enclosed with the inclusion of an intermediate space ( 9 ), used for feeding a liquid refrigerant through, by a cryostat (KR) which consists of two stainless steel tubes ( 6, 7 ) extending concentrically with one another and separated from one another by an intermediate space ( 8 ). In order to protect against abrasion of metallic parts, the surface of the screen (S) of the cable (KA), which is enclosed by the cryostat (KR), and/or of the cryostat (KR) is provided all around on its inner surface with a liner layer ( 10 ) made of an abrasion-resistant material with a lower friction coefficient compared with steel, which, when it encloses the screen (S) of the cable (KA), is permeable for the refrigerant.

RELATED APPLICATION

This application claims the benefit of priority from European PatentApplication No. 06291287.8, filed on Aug. 8, 2006, the entirety of whichis incorporated by reference.

DESCRIPTION

Field of the Invention

The invention relates to a system having a superconductive cable whichconsists of a superconductive inner conductor, a screen arrangedconcentrically therewith and a dielectric applied between the innerconductor and the screen, in which the screen is constructed from asuperconductive part and a part consisting of an electrically highlyconductive material enclosing the latter, and in which the screen isenclosed with the inclusion of an intermediate space, used for feeding aliquid refrigerant through, by a cryostat which consists of twostainless steel tubes extending concentrically with one another andseparated from one another by an intermediate space, which is evacuatedand provided with superinsulation.

BACKGROUND

A superconductive cable has electrical conductors made of a specialmaterial, which enters the superconductive state at sufficiently lowtemperatures. The electrical resistance of a correspondingly constructedconductor thereby tends towards zero. Suitable materials are for exampleYBCO (yttrium-barium-copper oxide) or BiSCCO(bismuth-strontium-calcium-copper oxide). Sufficiently low temperaturesfor such a material to achieve the superconductive state lie, forexample, between 67 K and 110 K. Suitable refrigerants are for examplenitrogen, helium, neon and hydrogen or mixtures of these substances,respectively in the liquid state.

US 2005/0056456 A1 discloses a superconductive cable having a centraltube for conveying a refrigerant. Two superconductive conductors, twoelectrostatic screens and a dielectric are arranged around the tube. Theouter-lying superconductive conductor as a return conductor is enclosedby a layer serving as mechanical protection, which is impermeable for arefrigerant. The cable is arranged in a cryostat consisting of twoconcentric tubes, between which there is insulation. Between the cableand the cryostat, there is a cavity for conveying a refrigerant.

The system described in the introduction comprises a superconductivecable, in which the refrigerant also penetrates into the dielectric asan impregnating medium during operation. Such a cable is referred to asa cold-dielectric cable. It is distinguished in that very high powerscan be transmitted in the high-voltage range. Such a cable consists ofan inner conductor and a screen or outer conductor arrangedconcentrically therewith, which are separated from each other and keptat a distance by a dielectric (insulation). The superconductiveconductors consist, for example, of strips of superconductive materialsuch as YBCO or BiSCCO, which are wound close together with a long pitcharound a support. The support for the inner conductor may be a tube orcord or strand made of electrically highly conductive material, whichalso serves to carry the electrical current in case of short circuit.The support, on the other hand, may also be made from a poorlyconductive or nonconductive metal if it is not deemed necessary to carrya short-circuit current in this element. The screen of the cable isconstructed from a superconductive part and a part—hereafter referred toas the “conductor” for brevity—enclosing the latter and also consistingof an electrically highly conductive material. The conductor in turnserves to carry the current in case of short circuit. For the screen,the dielectric serves as a support. It consists, for example, of amultiplicity of layers of paper and/or paper laminated withpolypropylene. Around the cable, for thermal insulation and to completethe system while including an air gap, a cryostat is arranged whichcomprises two stainless steel tubes lying inside one another, betweenwhich so-called superinsulation and a spacer are arranged. In the spacebetween the two tubes of the cryostat, there is a vacuum.

During operation of the system, a superconductive cable is cooled fromroom temperature to a temperature of for example 73 K. The cable thenshrinks by about 0.3%. A 600 m long cable thus shrinks by about 1.8 m.Owing to its special structure, on the other hand, the cryostat does notshrink during this cooling, or shrinks only insubstantially. Whencooling the cable, as well as when reheating it after “switching off”the cooling, a relative movement therefore takes place between thecryostat and the cable. The outer layer of the cable, i.e. theconductor, consists of an electrically highly conductive metal, forexample copper or aluminium. Both materials have a lower abrasionstrength compared with the inner tube of the cryostat. Metallicparticles therefore become abraded from the surface of the conductorduring the described relative movements. In regions of the cable orsystem which are exposed to electrical fields, for example terminations,these can lead to considerable problems even to the extent of electricalbreakdown, which could cause destruction of a termination. This risk isfurther exacerbated when the inner tube of the cryostat is corrugatedtransversely to its longitudinal direction, since increased abrasiontakes place because of the corrugation.

OBJECTS AND SUMMARY

It is an object of the invention to configure the system presented inthe introduction, so that no metallic particles generated by abrasioncan enter regions of the system which are exposed to electrical fields.

This object is achieved according to the invention

-   -   in that the surface of the screen of the cable, which is        enclosed by the cryostat, and/or of the cryostat is provided all        around on its inner surface with a liner layer made of an        abrasion-resistant material with a lower friction coefficient        compared with steel, and    -   in that a corresponding liner layer enclosing the screen of the        cable is permeable for the refrigerant.

The cable's conductor consisting of electrically highly conductivematerial is substantially protected against abrasion by the liner layer.In the event of relative movement between the cable and the cryostat, nometal particles are therefore abraded from the latter. At the same time,the movement of the cable when it contracts or expands in the cryostatis facilitated owing to the reduced friction between the two parts.

Bronze is advantageously used as the abrasion-resistant material for theliner layer, and preferably in the form of a strip which is wound aroundthe conductor of the cable with a gap.

BRIEF DESCRIPTION OF DRAWINGS:

Exemplary embodiments of the subject-matter of the invention arerepresented in the drawings, in which:

FIG. 1 shows a cross section through the system according to theinvention with a cold-dielectric superconductive cable.

FIG. 2 shows an embodiment of the system modified relative to FIG. 1.

DETAILED DESCRIPTION:

In the system represented in FIGS. 1 and 2, a cold-dielectricsuperconductive cable KA is respectively arranged in a cryostat KR. Thecable KA has an inner conductor 1 made of superconductive material,which is arranged around a metallic core 2 made of an electricallyhighly conductive metal, for example copper. The core 2 may be a cord orstrand. It may nevertheless be configured as a tube, through which arefrigerant can be conveyed during operation of the system. The innerconductor 1 is enclosed by a dielectric 3, which consists of a pluralityof layers of paper and/or paper laminated with polypropylene. The screenS of the cable KA, which consists of an inner-lying superconductive part4 and an outer-lying part 5 —hereafter referred to as the “conductor5”—enclosing the latter and consisting of an electrically highlyconductive metal, is arranged over the dielectric 3. The conductor 5preferably consists of copper.

The cryostat KR is constructed from two tubes 6 and 7 consisting ofstainless steel, which are separated from one another by an intermediatespaces 8. They may advantageously be corrugated transversely to theirlongitudinal direction, and they are preferably arranged coaxially withone another. The intermediate space 8 is evacuated and equipped withsuperinsulation and spacers, which hold the two tubes 6 and 7 in theirmutual position. The superinsulation may consist in a manner known perse of a plurality of layers of a plastic film evaporation-coated withaluminium.

The cable KA is arranged in the cryostat KR, and specifically whileleaving free an intermediate space 9 through which a pressurizedrefrigerant, for example nitrogen, is conveyed during operation of thesystem. According to requirements, the pressure of the refrigerant liesbetween 3 bar and 20 bar. In the exemplary embodiment represented inFIG. 1, the conductor 5 of the cable KA is enclosed by a liner layer 10made of abrasion-resistant material with a lower friction coefficientcompared with steel, which bears directly on the conductor 5. It ispermeable for the refrigerant, so that the latter can penetrate as animpregnating medium into the dielectric 3. The thickness of the linerlayer 10 is advantageously from 0.1 mm to 0.2 mm.

Suitable materials for the liner layer 10 are, for example,polytetrafluoroethylene and molybdenum sulphate. Bronze, however, isparticularly advantageously used for the liner layer 10. If the linerlayer 10 is applied onto the cable KA, it advantageously consists of abronze strip which is wound around the conductor 5 with a gap.

The liner layer 10 may also be applied according to FIG. 2 on the innersurface of the inner tube 7 of the cryostat KR. It is also possible toprovide a liner layer 10 both around the cable KA and in the tube 7.

Movements of the cable KA in the cryostat KR occur during correspondingcooling when putting the system into operation and during correspondingheating when switching the system off in case of faults and formaintenance. When using the liner layer 10 between the cable KA and thecryostat KR, no abrasion was any longer observed even after a sizeablenumber of such cooling and heating cycles.

1. System having a superconductive cable comprising: a superconductiveinner conductor; a screen arranged concentrically therewith; and adielectric applied between the inner conductor and the screen, in whichthe screen is constructed from a superconductive part and a part madefrom an electrically highly conductive material enclosing the latter,and in which the screen is enclosed with the inclusion of anintermediate space, used for feeding a liquid refrigerant through, by acryostat which has two stainless steel tubes extending concentricallywith one another and separated from one another by an intermediatespace, which is evacuated and provided with superinsulation, wherein thesurface of the screen of the cable, which is enclosed by the cryostat,and/or of the cryostat is provided all around on its inner surface witha liner layer made of an abrasion-resistant material with a lowerfriction coefficient compared with steel, and a corresponding linerlayer enclosing the screen of the cable is permeable for therefrigerant.
 2. System according to claim 1, wherein the liner layer ismade from bronze.
 3. System according to claim 1, wherein the linerlayer enclosing the screen of the cable includes a bronze strip woundaround the screen with a gap.
 4. System according to claim 1, wherein atleast the inner tube of the cryostat, facing the screen of the cable, iscorrugated transversely to its longitudinal direction.